1. Field of the Invention
The present invention relates to a process for producing an electric double layer capacitor having a low resistance and a large capacitance of at least 10 F.
2. Discussion of the Background
A conventional low resistance large capacitance electric double layer capacitor for power application is such that a pair of strip electrodes each having a thin film polarizable electrode material made mainly of active carbon powder formed on a current collector, are wound with a separator interposed, to form an element, this element is impregnated with an electrolyte and accommodated in a bottomed cylindrical metal container, and an opening of the container is sealed with a sealing member. Further, JP-A-4-154106 discloses an electric double layer capacitor wherein rectangular positive electrode and negative electrode each having a polarizable electrode formed on each side of a current collector, are alternately laminated in plurality with a separator interposed to form an element, the element is accommodated in an angular container, and the element is impregnated with an electrolyte and sealed by a sealing cover. For the electrodes of these electric double layer capacitors, activated carbon having a large specific surface area, is employed.
With conventional electric double layer capacitors employing activated carbon, a usual withstand voltage per unit element is from about 2.0 to 2.8 V in the case of electric double layer capacitors employing a non-aqueous electrolyte, although it may depend also on the selection of the solvent and the solute to be used. The energy is proportional to the square of the voltage. Accordingly, it is desired to increase the output density by increasing the energy density by improvement of the durability against application of a high voltage and by an increase of the capacitance density and by lowering the internal resistance, so that a larger quantity of energy can be taken out rapidly. Further, from the viewpoint of the energy storage, it is desired to improve the voltage-holding property after charging.
JP-A-7-022295 discloses that in a process for producing a coin-shaped electric double layer capacitor employing a non-aqueous electrolyte, a voltage of 2 V is preliminarily applied prior to sealing the container, whereby it is possible to suppress an increase of the thickness of the cell or an increase of the internal resistance with time, even if a voltage of 2 V is applied during charging. However, this electric double layer capacitor has a high internal resistance and a small capacitance, and further the voltage-holding property is also inadequate, and as such, it can not be applied to power application.
Further, JP-A-5-343263 discloses that polarizable electrodes are impregnated with a sulfuric acid electrolyte, a voltage of 1 V is preliminarily applied, then an inert gas is injected, followed by sealing, whereby it is possible to reduce the internal resistance and leakage current and to increase the capacitance. However, this electric double layer capacitor employs an aqueous electrolyte, whereby the useful voltage of a unit element is 1 V at the highest, the energy density is low and the voltage-holding property is inadequate, and therefore it is hardly applicable to power application.
Whereas, in JP-A-10-41199, the present inventors have proposed a method for obtaining a large capacitance electric double layer capacitor which is excellent in the voltage-holding property and which exhibits little decrease with time of the capacitance, by preliminarily applying a voltage slightly higher than the rated voltage.
Heretofore, to increase the capacitance of an electric double layer capacitor, activated carbon having a larger specific surface area has been employed, but there has been a problem that when a high voltage is applied, a gas is likely to be generated by decomposition of the surface functional groups of activated carbon or the solvent of the electrolyte, or by decomposition of impurities contained in a small amount in the capacitor cell. According to the method disclosed in JP-A-10-41199, a gas can be generated prior to the use of the electric double layer capacitor by a preliminary application of a voltage, and this gas can be discharged out of the capacitor container. However, if it is used for a long period of time by applying a voltage continuously for e.g. a few thousands hours, a gas will gradually be generated, to increase the inner pressure of the container, and especially when a readily deformable container such as angular container is employed, there will be a problem that the container tends to swell. Accordingly, it is necessary to take a deformation of the container into consideration, when a module is to be constituted by means of a plurality of capacitors.
Accordingly, it is an object of the present invention to provide a large capacitance electric double layer capacitor having a high withstand voltage, whereby the container is hardly deformable, and the performance is constant even when it is used for a long period of time.
The present invention provides a process for producing a large capacitance electric double layer capacitor, which comprises a step of forming an element by disposing a positive electrode and a negative electrode containing a carbon material having a specific surface area of at least 500 m2/g to face each other with a separator interposed, a step of impregnating the element with a non-aqueous electrolyte and then applying an applied voltage of from 1 to 1.5 times the rated voltage across the positive electrode and the negative electrode, a step of maintaining the element under a reduced pressure, a step of accommodating the element in a metal container, and a step of sealing the metal container.
this specification, the rated voltage is the maximum working voltage allowable for the guaranteed life as an electric double layer capacitor in a usual working condition. When the rated voltage is not prescribed for a product, a voltage is applied to the electric double layer capacitor at 60xc2x0 C. for 1000 hours, whereby the maximum voltage at which the volume change can be controlled within 30%, is taken as the rated voltage. Further, in this specification, the element represents one having a positive electrode and a negative electrode facing each other with a separator interposed, which is accommodated in one container for electric double layer capacitor. For example, in a case where a positive electrode and a negative electrode are accommodated in a container as alternately laminated in plurality with a separator interposed, the entire laminate is called an element.
In the process of the present invention, the applied voltage to the element is from 1 to 1.5 times the rated voltage of the electric double layer capacitor. If it is less than 1 time, the effect for improving the voltage holding property is little as compared with a case where no applied voltage is applied, and if it exceeds 1.5 times, the initial capacitance tends to be low, or the internal resistance tends to increase. Preferably, it is from 1 to 1.15 times, particularly preferably from 1.03 to 1.12 times.
The above application of the voltage is preferably carried out at a temperature of from 35 to 85xc2x0 C. When the voltage is applied while heating, the effect for improving the voltage-holding property increases, whereby the time for applying the voltage can be shortened. If it is lower than 35xc2x0 C., the heating effect is small, and if it exceeds 85xc2x0 C., the initial capacitance tends to be low, and the internal resistance tends to increase. Particularly preferred is from 50 to 70xc2x0 C.
The time for applying the above voltage is preferably at least 2 hours, usually from 5 to 100 hours. If the time for application of the voltage is short, the voltage-holding property can not be increased, and the amount of gas generated during use tends to be large, whereby swelling of the capacitor container tends to increase. On the other hand, if the time for application of the voltage is long, the productivity for the electric double layer capacitor tends to be poor.
If the voltage is applied in the production process as in the present invention, gases such as CO, CO2 and H2 will be generated in the case of an electric double layer capacitor using a usual carbon material having surface functional groups, such as activated carbon having a high specific surface area, for electrodes, and a non-aqueous electrolyte. Such gases are believed to be formed by decomposition of the surface functional groups of the carbon material or the solvent of the electrolyte, or by the decomposition of impurities contained in a small amount in the cell. The generated gases, such as CO2 gas, will be dissolved in the electrolyte, but if a gas which can not be fully dissolved, or a gas which can not be dissolved, is present, the inner pressure of the capacitor container will increase, and consequently, the container will be swelled. Therefore, in the present invention, it is preferred to apply the voltage to the element in a non-sealed state.
In the present invention, the capacitor element is maintained under a reduced pressure usually after the step of applying the voltage. Specifically, a container having the capacitor element accommodated therein, is brought to be under a reduced pressure. Here, the container may be a metal container for an electric double layer capacitor or other container capable of accommodating a capacitor element. When other container is to be used, a plurality of elements may be simultaneously maintained under a reduced pressure in one container.
By maintaining the element under a reduced pressure, the gas which can not fully be dissolved in the non-aqueous electrolyte will be purged from the capacitor element and out of the container accommodating the element. At that time, a gas present in voids of the electrodes or in voids of the separator will also be purged. At the same time, a gas such as CO2 dissolved in the electrolyte can also be purged, whereby no substantial gas may remain as dissolved in the electrolyte.
The majority of the gas generated during the use of the electric double layer capacitor is CO2 gas, which can be dissolved in the non-aqueous electrolyte. Accordingly, if the above application of the voltage and maintenance of the element under a reduced pressure are carried out, even if a gas is generated again during the use while the interior of the capacitor container is made to be the atmospheric pressure, this gas will firstly be dissolved in the electrolyte. Namely, when an electric double layer capacitor prepared by the process of the present invention, is used, even if a small amount of a gas is generated, an increase of the internal pressure of the container may be negligible, and the container will not be swelled. Accordingly, even if the electric double layer capacitor is used for a long period of time, the internal pressure scarcely increases, and the container will scarcely be swelled.
In the present invention, the interior of the container accommodating the capacitor element is preferably made to be under a reduced pressure of at most 160 mmHg, in order to maintain the above-mentioned reduced pressure. If the pressure is not sufficiently reduced, the gas dissolved in the electrolyte can not be purged. It is particularly preferred to bring the interior to a reduced pressure of at most 10 mmHg. Further, the time for maintaining it under a reduced pressure is preferably from 5 to 120 minutes, so that the gas dissolved in the electrolyte will sufficiently be purged.
Further, in the present invention, the step of applying an applied voltage of from 1 to 1.5 times the rated voltage across the positive electrode and the negative electrode and the step of maintaining the element under a reduced pressure, may be carried out simultaneously in one step. Namely, an impressed voltage may be applied to the element under a reduced pressure. By this method, the gas will be removed out of the element as it is generated.
In the present invention, it is preferred that after maintaining the electric double layer capacitor element under a reduced pressure, the pressure in the interior of the metal container is adjusted to be the atmospheric pressure at the stage of sealing the metal container having the element accommodated therein. The interior of the metal container may not completely be the atmospheric pressure so long as it is adjusted to be substantially the atmospheric pressure. Even when the element is accommodated in the metal container and maintained to be under a reduced pressure, it is preferred to bring the interior of the container to be substantially the atmospheric pressure by injecting an inert gas which does not dissolve in the non-aqueous electrolyte and which does not react with the electrodes. In order to seal the container while maintaining the interior of the container under a reduced pressure, a complex sealing installation will be required, and a complex production process will be required.
For the electric double layer capacitor in the present invention, a non-aqueous solvent is used as the solvent for the electrolyte in order to increase the withstand voltage. Specifically, at least one solvent selected from the group consisting of propylene carbonate, ethylene carbonate, butylene carbonate, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, 1,2-dimethoxyethane, sulfolane and methylsulfolane, is preferred from the viewpoint of the chemical and electrochemical stability, the electrical conductivity and the low temperature characteristics. Among them, when a carbonate type solvent is used mainly, the effects of the present invention will be large.
Further, the solute of the electrolyte is preferably a salt having a quaternary ammonium ion of the formula R1R2R3R4N+ or a quaternary phosphonium ion of the formula R1R2R3R4P+ (wherein each of R1, R2, R3 and R4 which are independent of one another, is a C1xe2x88x924 alkyl group) combined with an anion of BF4xe2x88x92, N(CF3SO2)2xe2x88x92, PF6xe2x88x92 or ClO4xe2x88x92. Specifically, (C2H5)3(CH3)NBF4, (C2H5)4NBF4 or (C2H5)3(CH3)PBF4 is, for example, preferred.
When the above-described electrolyte is used, the withstand voltage of the electrolyte is high, whereby the rated voltage can be made to be from 2.5 to 3.3 V. And, the impressed voltage to be applied in the production process to an element of an electric double layer capacitor having a rated voltage within this range, is preferably from 2.5 to 3.5 V.
The electrodes of the electric double layer capacitor in the present invention are electrodes containing a carbon material having a specific surface area of at least 500 m2/g with respect to both the positive electrode and the negative electrode. As such a carbon material, activated carbon, polyacene or carbon black may, for example, be specifically mentioned, and it is particularly preferred that the electrode is composed of activated carbon, a conductive material such as carbon black which imparts electrical conductivity and a binder, and it is bonded to a metal current collector. Such an electrode may be formed, for example, in such a manner that a certain solvent is added and mixed to the carbon material, the carbon black and the binder to obtain a slurry, which is coated or impregnated to the metal current collector, followed by drying and, if necessary by pressing to integrate it with the current collector.
The above carbon material is particularly preferably such that the average particle size is at most 30 xcexcm, and the specific surface area is from 1,200 to 3,000 m2/g, whereby the capacitance of an electric double layer capacitor can be made large, and the internal resistance can be made low.
As the binder to be contained in the electrodes, polyvinylidene fluoride, a copolymer comprising polymer units based on a fluoroolefin and polymer units based on other monomer, carboxymethyl cellulose, polyvinylpyrrolidone, polyvinyl alcohol, polyacrylic acid or polyimide, is, for example, preferred. A crosslinking agent, etc., may further be added, as the case requires. Further, as the solvent for the slurry, one capable of dissolving the binder is preferred, and n-methylpyrrolidone, water, dimethylformamide, toluene, xylene, methyl ethyl ketone, ethyl acetate, methyl acetate, dimethyl phthalate, ethanol, methanol or butanol, may, for example, be mentioned.
Further, a method is also preferred wherein the carbon material, the conductive material powder and a fluorine resin powder such as polytetrafluoroethylene, as the binder, are kneaded by an addition of a solvent, followed by rolling to obtain a sheet, which is electrically bonded to a metal current collector via a conductive adhesive, to form an electrode. If an electrode obtainable by this method is employed, it is possible to obtain an electric double layer capacitor having a high capacitance density, such being preferred.
In the present invention, in order to obtain an electric double layer capacitor having a large capacitance of at least 10 F, a structure capable of enlarging the electrode area is preferred, such as a cylindrical type wherein a pair of strip electrodes are wound to face each other with a separator interposed, and accommodated in a cylindrical container, or an angular type wherein a plurality of positive electrodes and negative electrodes are alternately laminated with a separator interposed to form an element, which is accommodated in an angular container.
The metal current collector to be electrically connected with the electrode may be one which is resistant to the electrolyte, and for example, stainless steel or aluminum is preferably employed. The shape of the metal current collector may be any shape such as a foil-shape, an expanded metal-shape, a fiber-sintered sheet shape or a plate metal foam. Among them, a current collector made of an aluminum foil having a thickness of from 20 to 100 xcexcm is preferred, since the winding or laminating step is easy. When a metal foil is used for a current collector, it is particularly preferred to roughen the surface by chemical, electrochemical or physical etching, whereby adhesion between the activated carbon electrode layer and the metal foil will be improved, and the electrical resistance can be made low.
In the present invention, as the separator to be interposed between the positive electrode and the negative electrode, a glass fiber mat, a cellulose paper made of manila hemp or craft, a hydrophilic porous polytetrafluoroethylene film or a polypropylene non-woven fabric may, for example, be mentioned. Further, as the container to be used in the present invention, a metal container made of aluminum, stainless steel, iron or an alloy thereof, is particularly preferred.
In the present invention, it is preferred that by using preliminarily heated and dried sheet-form electrodes as the positive electrode and the negative electrode, the positive electrode and the negative electrode are wound or laminated to face each other with a separator interposed to form an element, which is vacuum-dried at a temperature of from 120 to 250xc2x0 C. to remove a volatile component such as moisture in the element, and then the electrolyte is vacuum-impregnated. It is preferred to adjust the ambient temperature during impregnation at a level of from 40 to 80xc2x0 C., whereby the viscosity of the electrolyte will be low, and the electrolyte will readily be impregnated to the electrodes. The impregnation of the electrolyte may be carried out either after accommodating the element in the metal container or prior to accommodating the element in the metal container.
Now, the present invention will be specifically described with reference to Working Examples (Examples 1 and 2) and Comparative Examples (Examples 3 to 6). However, the present invention is not limited thereto.